Fuel cell system and method for controlling same

ABSTRACT

A fuel cell system  100  includes: a fuel cell  1  for generating a power by causing an electrochemical reaction between an oxidant gas supplied to an oxidant electrode  34  and a fuel gas supplied to a fuel electrode  67;  a fuel gas supplier HS for supplying the fuel gas to the fuel electrode  67;  and a controller  40  for controlling the fuel gas supplier HS to thereby supply the fuel gas to the fuel electrode  67,  the controller  40  being configured to implement a pressure change when an outlet of the fuel electrode  67  side is closed, wherein based on a first pressure change pattern for implementing the pressure change at a first pressure width ΔP1, the controller  40  periodically changes a pressure of the fuel gas at the fuel electrode  67.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/129,986, filed Aug. 2, 2011, which is the National Stage ofApplication No. PCT/JP2009/069425 filed Nov. 16, 2009, which is basedupon and claims the benefit of priority from prior Japanese ApplicationNo. 2008-298191, filed Nov. 21, 2008 and Japanese Application No.2008-302465, filed Nov. 27, 2008; the entire contents of all of whichare incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a fuel cell system.

BACKGROUND ART

Conventionally, such a fuel cell system is known as is provided with afuel cell where a fuel gas (for example, hydrogen) is supplied to a fuelelectrode and an oxidant gas (for example, air) is supplied to anoxidant electrode to thereby make an electrochemical reaction of thesegases, thus implementing a power generation.

With respect to the fuel cell system of the above type, nitrogenincluded in the air is permeated to the fuel electrode side, so that thefuel electrode has a portion having a high nitrogen concentration, thatis, a portion having a low hydrogen concentration. The thus caused gasunevenness is a cause for deteriorating members included in the fuelcell. Then, Patent Literature 1 discloses a method of changing gaspressures of the fuel electrode and oxidant electrode to thereby purgethe water of the fuel cell and the accumulated unreactive gas.

CITATION LIST Patent Literature

[Patent Literature 1]

Japanese Patent Publication No. 2007-517369 {JP2007517369(T)}

SUMMARY OF INVENTION Technical Problem

However, with respect to the method disclosed in the Patent Literature1, a pressure change with a relatively large pressure width is necessaryfor purging the liquid water and unreactive gas. Thereby, a large stressmay be applied to electrolyte membranes included in the fuel cell, thuscausing such a possibility as may deteriorate durability of the fuelcell.

The present invention has been made in view of the above circumstances.It is an object of the present invention to suppress unevenness ofreactive gas while suppressing durability deterioration of the fuelcell.

Moreover, it is another object of the present invention to suppress thestress caused in the fuel cell or fuel gas supply components to therebysuppress deterioration of the fuel cell system.

Solution to Problem

A fuel cell system according to an aspect of the present inventioncomprises: a fuel cell for generating a power by causing anelectrochemical reaction between an oxidant gas supplied to an oxidantelectrode and a fuel gas supplied to a fuel electrode; a fuel gassupplier for supplying the fuel gas to the fuel electrode; and acontroller for controlling the fuel gas supplier to thereby supply thefuel gas to the fuel electrode, the controller being configured toimplement a pressure change when an outlet of the fuel electrode side isclosed, wherein based on a first pressure change pattern forimplementing the pressure change at a first pressure width, thecontroller periodically changes a pressure of the fuel gas at the fuelelectrode.

A method of controlling a fuel cell system according to the aspect ofthe present invention comprises: generating a power by causing anelectrochemical reaction between an oxidant gas supplied to an oxidantelectrode and a fuel gas supplied to a fuel electrode; supplying thefuel gas to the fuel electrode; and controlling the supplying operationof the fuel gas to thereby supply the fuel gas to the fuel electrode,and implementing a pressure change when an outlet of the fuel electrodeside is closed, wherein based on a first pressure change pattern forimplementing the pressure change at a first pressure width, thecontrolling operation periodically changes a pressure of the fuel gas atthe fuel electrode.

A fuel cell system according to the aspect of the present inventioncomprises: a fuel cell for generating a power by causing anelectrochemical reaction between an oxidant gas supplied to an oxidantelectrode and a fuel gas supplied to a fuel electrode; a fuel gassupplying means for supplying the fuel gas to the fuel electrode; and ameans for controlling the fuel gas supplying means to thereby supply thefuel gas to the fuel electrode, the controlling means being configuredto implement a pressure change when an outlet of the fuel electrode sideis closed, wherein based on a first pressure change pattern forimplementing the pressure change at a first pressure width, thecontrolling means periodically changes a pressure of the fuel gas at thefuel electrode.

Advantageous Effects of Invention

According to the present invention, periodically changing a pressure ofa fuel gas at a fuel electrode based on the first pressure changepattern which implements pressure change at the first pressure width canagitate the fuel electrode side gas. With this, the fuel electrode sidegas can be made even.

Moreover, according to the present invention, the fuel gas supplyquantity in the implementation period of one control pattern isincreased, thus it is possible to suppress increase in the number ofimplementations of the pressure rise-fall per unit period. With this, astress applied to the fuel cell or fuel gas supply components can berelieved, thus it is possible to suppress deterioration of the fuel cellsystem.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1( a) is a block diagram schematically showing a structure of thefuel cell system according to the first embodiment. FIG. 1( b) is ablock diagram schematically showing another structure of the fuel cellsystem according to the first embodiment.

FIG. 2( a) is explanatory view showing a state of hydrogen on the fuelelectrode side in the fuel cell, showing hydrogen streamlines in thefuel electrode side gas flow channel. FIG. 2( b) shows the hydrogenconcentration distribution in the fuel electrode side gas flow channel.FIG. 2( c) shows the hydrogen concentration distribution on the fuelelectrode side reaction surface.

FIG. 3 (a) is an explanatory view schematically showing the fuel cell,assuming eight current measurement points. FIG. 3( b) shows time-seriestransition of the current distribution at an individual measurementpoint.

FIG. 4 is a cross sectional view schematically showing the structure ofthe fuel cell.

FIG. 5 is an explanatory view showing a leak nitrogen quantity relativeto nitrogen partial pressure difference between the oxidant electrodeand the fuel electrode.

FIG. 6 is an explanatory view showing the relation between an ambienthumidity and a leak nitrogen quantity according to an ambienttemperature.

FIG. 7( a) is an explanatory view schematically showing an agitationstate of hydrogen with the unreactive gas. FIG. 7( b) shows a timing forstopping the hydrogen supply (valve closing operation).

FIG. 8( a) is an explanatory view showing a liquid water dischargestate. FIG. 8( b) shows a timing for stopping the hydrogen supply (valveclosing operation). FIG. 8( c) shows another example of the timing forstopping the hydrogen supply (valve closing operation). FIG. 8( d) showsstill another example of the timing for stopping the hydrogen supply(valve closing operation).

FIG. 9 is an explanatory view showing current distribution in the powergeneration surface.

FIG. 10 is a flowchart showing process procedures of a method ofcontrolling the fuel cell system according to the second embodiment.

FIG. 11 is an explanatory view showing control patterns by the firstcontrol method.

FIG. 12 is an explanatory view showing control patterns by the secondcontrol method.

FIG. 13 is an explanatory view showing control patterns by the thirdcontrol method.

FIG. 14 is an explanatory view showing a transition of pressurerise-fall in the fuel electrode.

FIG. 15 is an explanatory view of the first keeping time Tp1.

FIG. 16 is an explanatory view of the second keeping time T.

FIG. 17 is an explanatory view showing the load relative to each of thefirst keeping time Tp1 and the second keeping time Tp2.

FIG. 18 is an explanatory view showing the load relative to each of thefirst keeping time Tp1 and the second keeping time Tp2.

FIG. 19 is an explanatory view showing the upper limit pressure P1 andlower limit pressure P2 relative to the load current.

FIG. 20( a) is an explanatory view schematically showing the fuelelectrode side capacity Rs in the fuel cell stack and the capacity Rt ofthe capacity portion. FIG. 20 (b) shows that new hydrogen flowed intothe fuel cell stack in an amount of around ¼ of the capacity of the fuelsystem.

FIG. 21 is an explanatory view of the upper limit pressure P1 and lowerlimit pressure P2.

FIG. 22 is an explanatory view of a pressure fall speed.

DESCRIPTION OF EMBODIMENTS First Embodiment

FIG. 1( a) is a block diagram schematically showing a structure of afuel cell system 100 according to the first embodiment of the presentinvention. The fuel cell system 100 is installed, for example, in avehicle that is a mobile object, where the vehicle is driven by anelectric power supplied from the fuel cell system 100.

The fuel cell system 100 is principally provided with a fuel cell stack1 including a plurality of stacked fuel cells. Each of the fuel cellsincluded in the fuel cell stack 1 is so formed that a fuel cellstructure is sandwiched between a pair of separators, where the fuelcell structure has such a structure that a fuel electrode 67 (refer toafter-described FIG. 4) and an oxidant electrode 34 (refer toafter-described FIG. 4) sandwich therebetween a solid polymerelectrolyte membrane.

In the fuel cell stack 1, corresponding to each of the fuel gas and theoxidant gas, a pair of internal flow channels are so formed as to extendin a stack direction of the fuel cell. Of the pair of the internal flowchannels (manifolds) corresponding to the fuel gas; with respect to asupply internal flow channel as the first internal flow channel, a fuelgas is supplied to each of the fuel electrode 67 side reaction surfacesvia the fuel electrode 67 side gas flow channels (cell flow channels) ofthe individual fuel cells, while with respect to a discharge internalflow channel as the second internal flow channel, a gas (hereinafterreferred to as “fuel electrode off-gas”) discharged from each of thefuel electrode 67 side gas flow channels of the individual fuel cellsflows into the discharge internal flow channel. Likewise, of the pair ofthe internal flow channels corresponding to the oxidant gas; withrespect to a supply internal flow channel as the first internal flowchannel, an oxidant gas is supplied to each of the oxidant electrode 34side reaction surfaces via the oxidant electrode 34 side gas flowchannels (cell flow channels) of the individual fuel cells, while withrespect to a discharge internal flow channel as the second internal flowchannel, a gas (hereinafter referred to as “oxidant electrode off-gas”)discharged from each of the oxidant electrode 34 side gas flow channelsof the individual fuel cells flows into the discharge internal flowchannel. The fuel cell stack 1 according to the first embodiment adoptswhat is called a counter flow method where the fuel gas and the oxidantgas flow in directions opposite to each other.

In each of the individual cells of the fuel cell stack 1,electrochemically reacting the fuel gas and the oxidant gas with eachother, which gases are respectively supplied to the fuel electrode 67and the oxidant electrode 34, generates an electric power.

According to the first embodiment, an explanation is made based on thecase of using hydrogen as a fuel gas and air as an oxidant gas. Inaddition, in this specification, the languages “fuel cell,” “fuelelectrode” and “oxidant electrode” are not to be used only fordesignating a single fuel cell or its fuel electrode or oxidantelectrode, but are also to be used for unanimously designating each ofthe fuel cells of the fuel cell stack 1 or their fuel electrodes oroxidant electrodes.

The fuel cell system 100 further includes a hydrogen system forsupplying hydrogen to the fuel cell stack 1 and an air system forsupplying air to the fuel cell stack 1.

In the hydrogen system, hydrogen as the fuel gas is stored in the fueltank 10 (for example, a high pressure hydrogen cylinder), and issupplied from the fuel tank 10 to the fuel cell stack 1 via a hydrogensupply flow channel (fuel electrode inlet flow channel) L1.Specifically, the hydrogen supply flow channel L1 has the first endportion connected to the fuel tank 10 and the second end portionconnected to an inlet side of the fuel gas supply internal flow channelof the fuel cell stack 1. In the hydrogen supply flow channel L1, a tanksource valve (not shown in FIG. 1) is disposed at a downstream of thefuel tank 10. Rendering the tank source valve in an open state allowsthe high pressure hydrogen gas from the fuel tank 10 to be mechanicallypressure-reduced to a predetermined pressure by means of apressure-reducing valve (not shown in FIG. 1) disposed at the downstreamof the fuel tank 10. The thus pressure-reduced hydrogen gas is furtherpressure-reduced by means of a hydrogen pressure adjusting valve 11disposed at the further downstream of the pressure-reducing valve, andthen is supplied to the fuel cell stack 1. The hydrogen pressuresupplied to the fuel cell stack 1, that is, the hydrogen pressure in thefuel electrode 67 can be adjusted by controlling opening degree of thehydrogen pressure adjusting valve 11. According to the first embodiment,the fuel tank 10, the hydrogen supply flow channel L1 and the hydrogenpressure adjusting valve 11 which is disposed in the hydrogen supplyflow channel L1 constitute a hydrogen supplier HS (fuel gas supplier HS)for supplying hydrogen to the fuel electrode 67 of the fuel cell stack1.

According to the first embodiment, the fuel cell stack 1 has such astructure that an outlet side of the fuel gas discharge internal flowchannel is basically closed, thus restricting the fuel electrodeoff-gas's discharge from the fuel cell stack 1, that is, the fuel cellstack 1 is included in the fuel cell system 100 which adopts what iscalled a closed system. Herein, the closed system does not mean an exactclosed state. For discharging, from the fuel electrode 67, impuritiessuch as inactive gas (nitrogen and the like) and liquid water, there isdisposed, as an exception, a discharge system capable of opening theoutlet side of the fuel gas discharge internal flow channel.Specifically, a fuel electrode off-gas flow channel (discharge flowchannel) L2 is connected to the outlet side of the fuel gas dischargeinternal flow channel. The fuel electrode off-gas flow channel L2 hasthe second end portion connected to an after-described oxidant electrodeoff-gas flow channel L6.

In the fuel electrode off-gas flow channel L2, a capacity portion(capacity device) 12 having a predetermined capacity Rs (seeafter-described FIG. 20) as a space is disposed, where the predeterminedcapacity Rs is, for example, equivalent to or about 80% of the fuelelectrode 67 side capacity with respect to all fuel cells included inthe fuel cell stack 1. The capacity portion 12 functions as a buffer forprimarily storing impurities included in the fuel electrode off-gasentering from the fuel electrode 67 side. In FIG. 1, a discharge waterflow channel L3 having an open first end portion is connected to thecapacity portion 12's lower portion in a vertical direction, and adischarge water valve 13 is provided for the discharge water flowchannel L3. The impurities (mainly, liquid water) contained in the fuelelectrode off-gas entering the capacity portion 12 is stored in thelower part of the capacity portion 12. Controlling the open-closed stateof the discharge water valve 13 can discharge the thus storedimpurities. Moreover, in the fuel electrode off-gas flow channel L2, apurge valve (shutter) 14 is disposed on a downstream of the capacityportion 12. The fuel electrode off-gas entering the capacity portion 12,specifically, the gas including the impurities (mainly, inactive gassuch as nitrogen) and unreacted hydrogen can be discharged bycontrolling the open-closed state of the purge valve 14.

The fuel electrode off-gas flow channel (discharge flow channel) L2, thecapacity portion (capacity device) 12 and the purge valve (shutter) 14form a limiter 70.

Meanwhile, the air as the oxidant gas of the air system is to be setforth. For example, air is compressed when an atmosphere is taken in bymeans of a compressor 20, thereby supplying the air to the fuel cellstack 1 by way of an air supply flow channel L5. The air supply flowchannel L5 has the first end portion connected to the compressor 20 andthe second end portion connected to the inlet side of an oxidant gassupply internal flow channel of the fuel cell stack 1. Moreover, an airsupply flow channel L5 has a humidifier 21 for humidifying the airsupplied to the fuel cell stack 1.

In the fuel cell stack 1, an oxidant electrode off-gas flow channel L6is connected to the outlet side of the oxidant gas discharge internalflow channel. With this, the oxidant electrode off-gas from the oxidantelectrode 34 in the fuel cell stack 1 is discharged outside by way ofthe oxidant electrode off-gas flow channel L6. The oxidant electrodeoff-gas flow channel L6 has the above-described humidifier 21, thusremoving the water generated by the generation (this removed water isused for humidifying the supply air). Moreover, in the oxidant electrodeoff-gas flow channel L6, an air pressure adjusting valve 22 is disposedon the downstream of the humidifier 21. Adjusting the opening degree ofthe air pressure adjusting valve 22 can control the air pressuresupplied to the fuel cell stack 1, that is, the air pressure of theoxidant electrode 34. According to the first embodiment, the compressor20, the air supply flow channel L5, and the air pressure adjusting valve22 which is disposed in the oxidant electrode off-gas flow channel L6constitute an oxidant gas supplier OS for supplying the air to theoxidant electrode 34 of the fuel cell stack 1.

Moreover, an output takeout device 30 for controlling an output (forexample, current) taken out from the fuel cell stack 1 is connected tothe fuel cell stack 1. By way of the output takeout device 30, the powergenerated in the fuel cell stack 1 is supplied, for example, to avehicle-driving electric motor (not shown in FIG. 1), a secondarybattery and various accessories necessary for the generation operationof the fuel cell stack 1. Moreover, the power generated by the outputtakeout device 30 is also supplied to the secondary battery (not shownin FIG. 1). This secondary battery is provided for supplementingshortage of the power supplied from the fuel cell stack 1 in suchoccasions as to start the fuel cell system 100 or in a transientresponse of the fuel cell system 100.

A controller (control device) 40 functions to administratively controlthe entire fuel cell system 100. By operating according to a controlprogram, the controller 40 controls operation conditions of the fuelcell system 100. A microcomputer including main components such as CPU,ROM, RAM and I/O interface can be used as the controller 40. Accordingto the control program stored in the ROM, the controller 40 implementsvarious calculations. Then, to various actuators (not shown in FIG. 1),the controller 40 outputs such calculation results as control signals.With this, the controller 40 controls various elements such as thehydrogen pressure adjusting valve 11, the discharge water valve 13, thepurge valve 14, the compressor 20, the air pressure adjusting valve 22and the output takeout device 30, to thereby implement the generationoperation of the fuel cell stack 1.

For detecting conditions of the fuel cell system 100, sensor signalsfrom various sensors and the like are input to the controller 40.According to the first embodiment, the above various sensors include ahydrogen pressure sensor 41, an air pressure sensor 42, and a stacktemperature sensor 43. The hydrogen pressure sensor 41 detects thehydrogen pressure supplied to the fuel cell stack 1, the air pressuresensor 42 detects the air pressure supplied to the fuel cell stack 1,and the stack temperature sensor 43 detects the temperature of the fuelcell stack 1.

According to the first embodiment, the controller 40 controls the fuelcell system 100 in the following manner. Firstly, the controller 40supplies air and hydrogen to the fuel cell stack 1, to thereby implementthe generation by the fuel cell stack 1. The pressure (operationpressure) of each of the air and the hydrogen which are supplied to thefuel cell stack 1 is set in advance either at a certain standard valuewhich is constant irrespective of operation load or at variable valueswhich are variable according to the operation load. Then, the controller40 supplies the air and hydrogen at a predetermined operation pressure,to thereby implement the generation of the fuel cell stack 1. Herein, asone feature of the first embodiment, when supplying the hydrogen to thefuel electrode 67 of the fuel cell stack 1, the controller 40periodically changes the hydrogen pressure in the fuel electrode 67 ofthe fuel cell stack 1, based on the first pressure change pattern forimplementing the pressure change at the first pressure width(differential pressure) and the second pressure change pattern forimplementing the pressure change at the second pressure width(differential pressure) larger than the first pressure width.Specifically, the controller 40 repeatedly implements basic controlpatterns, that is, a plurality of the first pressure change patterns,followed by the second pressure change pattern. When implementing thepressure change, the controller 40 stops hydrogen supply to the fuelcell stack 1, and on the condition that the hydrogen pressure in thefuel electrode 67 of the fuel cell stack 1 is decreased by thepredetermined pressure width (first pressure width or second pressurewidth), the controller 40 restarts the hydrogen supply to the fuel cellstack 1, to thereby allow the hydrogen pressure in the fuel electrode 67of the fuel cell stack 1 to return to the operation pressure. Openingand closing of the hydrogen pressure adjusting valve 11 accomplish thestop and restart of the hydrogen supply to the fuel cell stack 1.Referring to the value detected by the hydrogen pressure sensor 41 canmonitor the hydrogen pressure drop which is equivalent to the pressurewidth.

Moreover, FIG. 1( b) is a block diagram schematically showing anotherstructure of the fuel cell system 100 according to the first embodimentof the present invention. Herein, the structure abolishes the dischargewater valve 13, leaving the purge valve 14 only. With the abovestructure, controlling the open-close condition of the purge valve 14can discharge the gas included in the fuel electrode off-gas, that is,the gas including the impurities (mainly, inactive gas such as nitrogen,and liquid water) and unreacted hydrogen.

Hereinafter, concept of the fuel cell system 100 adopting the abovestructure and control method is to be set forth.

In view of improved fuel economy and decrease of driving power ofvarious accessories for operating the fuel cell stack, operating thefuel cell system 100 at a low stoichiometric ratio (otherwise referredto as “low reactive gas supply excess ratio”) and at a low flow ratelowers the flow velocity of the reactive gas (hydrogen or air) flowingin the gas flow channel (cell flow channel) in each of the fuel cells ofthe fuel cell stack 1. With this, impurities unnecessary for thegeneration reaction, for example, liquid water or an unreactive gas(mainly, nitrogen) are likely to be accumulated in the gas flow channel,which may prevent distribution of the reactive gas necessary for thegeneration. In this case, the output of the fuel cell stack 1 is loweredand the generation is disabled, in addition, the catalyst necessary forreaction may possibly be deteriorated.

For example, a condition for the fuel cell stack 1 to implement thegeneration by the following operations is to be taken into account:supplying air to the oxidant electrode 34 of the fuel cell stack 1;restricting the fuel electrode off-gas's discharge from the fuel cellstack 1; and constantly supplying hydrogen by an amount equivalent tohydrogen consumed in the fuel electrode 67. In the individual fuel cell,nitrogen in air makes a cross leak to the fuel electrode 67 side gasflow channel from the oxidant electrode 34 side gas flow channel by wayof the solid polymer electrolyte membrane included in the fuel cell.Meanwhile, to the fuel electrode 67 side gas flow channel, hydrogen inequivalent to hydrogen consumed by the generation reaction flows byconvection current. However, since the outlet side of the fuel gasdischarge internal flow channel is closed, the thus cross-leakednitrogen is pushed into the downstream side (outlet side) of the gasflow channel by the convection of hydrogen. Nitrogen of the fuelelectrode 67 is not consumed by the generation reaction. On top of that,nitrogen leak from the oxidant electrode 34 continuously increases thenitrogen in the fuel electrode 67 until the oxidant electrode 34 sidepartial pressure is equal to the fuel electrode 67 side partialpressure.

FIG. 2( a) to FIG. 2( c) are explanatory views showing states of thefuel electrode 67 side hydrogen in the fuel cell. FIG. 2( a) showshydrogen streamlines in the fuel electrode 67 side gas flow channel.Herein, the abscissa axis denotes a distance (in gas flow channeldirection) of the gas flow channel, where the left side of the abscissaaxis corresponds to the inlet side of the gas flow channel and the rightside of the abscissa axis corresponds to the outlet side of the gas flowchannel. Meanwhile, the ordinate axis denotes a height of the gas flowchannel, where the lower side of the ordinate axis corresponds to thereaction surface. Moreover, FIG. 2( b) shows hydrogen concentrationdistribution in the fuel electrode 67 side gas flow channel. Like FIG.2( a), the abscissa axis denotes the distance (in gas flow channeldirection) of the gas flow channel, while the ordinate axis denotes theheight of the gas flow channel. In FIG. 2( b), an area al denotes ahydrogen concentration range of 93% to 100%, an area a2 denotes thehydrogen concentration range of 83% to 93%, and an area a3 denotes thehydrogen concentration range of 73% to 83%. Moreover, an area a4 denotesthe hydrogen concentration range of 63% to 73%, an area a5 denotes thehydrogen concentration range of 53% to 63%, an area a6 denotes thehydrogen concentration range of 43% to 53%, and an area a7 denotes thehydrogen concentration range of 33% to 43%. Moreover, FIG. 2( c) showsthe hydrogen concentration distribution on the fuel electrode 67 sidereaction surface. Herein, the abscissa axis denotes the distance of thegas flow channel, where the left side of the abscissa axis correspondsto the inlet side of the gas flow channel while the right side of theabscissa axis corresponds to the outlet side of the gas flow channel.Meanwhile, the ordinate axis denotes the hydrogen concentration.

As stated above, the cross leaked nitrogen's inflow and the inflowhydrogen allow the fuel electrode 67 to have a portion where thenitrogen concentration is high, i.e., a portion where the hydrogenconcentration is low. Specifically, in the fuel cell, the furtherdownstream side (outlet side) of the gas flow channel has a tendency tofurther decrease the hydrogen concentration. Moreover, continuing thegeneration from such a state further decreases the hydrogenconcentration of the portion where the hydrogen concentration is low.

FIG. 3 is an explanatory view schematically showing the fuel cell. Asshown in FIG. 3( a), along the flow of the reactive gas, eight currentmeasurement points #1 to #8 are respectively assumed in the powergeneration surface of the fuel cell. FIG. 3( b) shows time-seriestransition of the current distribution at the individual measurementpoint #1 to #8. Specifically, as denoted by a broken line arrow, thecurrent distribution transition in each of the measurement points #1 to#8 is shifted from the one-dot chain line to the broken line and to thesolid line. That is, in the initial generation step, the hydrogenconcentration in the gas flow channel is substantially even, therefore,as denoted by the one-dot chain line, the current values at themeasurement points #1 to #8 are substantially equal to each other.However, continuously implementing the generation decreases the hydrogenconcentration on the outlet side of the gas flow channel, thus, asdenoted by the broken line or the solid line, the current values on theoutlet side of the gas flow channel drop and a current concentration iscaused on the inlet side of the gas flow channel. In such states, it isdifficult to continue the stable generation and the generation maypossibly be finally disabled. Moreover, since the above local currentdrop is difficult to detect, as the case may be, the output from thefuel cell stack is continuously taken with the current drop unnoticed.

FIG. 4 is a cross sectional view schematically showing the structure ofthe fuel cell. The fuel cell structure 150 included in the fuel cell hassuch a structure that the solid polymer electrolyte membrane 2 issandwiched between the fuel electrode 67 and the oxidant electrode 34which two electrodes (reactive electrodes) are pairwise. The solidpolymer electrolyte membrane 2 includes, for example, an ion conductivemacromolecular membrane such as a fluorine resin ion exchange membrane,and functions as an ion conductive electrolyte membrane through watersaturation. The oxidant electrode 34 includes a platinum-based catalyticlayer 3 carrying thereon a catalyst such as platinum and a gas diffusionlayer 4 including a porous body such as carbon fiber. The electrode 67includes a platinum-based catalytic layer 6 carrying thereon a catalystsuch as platinum and a gas diffusion layer 7 including a porous bodysuch as a carbon fiber. Moreover, the separators (not shown in FIG. 4)sandwiching therebetween the fuel cell structure 150 from both sidesrespectively have gas flow channels 5, 8 for supplying the reactivegases (hydrogen and air) to the individual reactive electrodes.

When the generation is continued, oxygen simultaneously with nitrogenleak from the oxidant electrode 34 side to the fuel electrode 67 side,thereby oxygen moves to the fuel electrode 67 side. Moreover, watergenerated by the generation reaction is present in the oxidant electrode34 side. Moreover, the gas diffusion layer 4 or the separator (not shownin Fig.), that is, the members included in the gas flow channel in thefuel cell or the members for supporting the catalyst mainly includecarbon. With this, the following reactions are promoted in the area(area B in FIG. 4) where the hydrogen is in short supply:

Fuel electrode 67 side: O₂+4H⁺+4e ⁻→2H₂O

Oxidant electrode 34 side: C+2H₂O→CO₂+4H⁺+4e ⁻  [Equation 1]

Referring to the equation 1, carbon in the structure of the fuel cellreacts with water generated on the oxidant electrode 34 side, to therebygenerate carbon dioxide on the oxidant electrode 34 side. This signifiesthat the structure in the fuel cell is eroded. Carbon included in eachof an element forming the gas flow channel, a structure carrying thereona catalyst for causing the reaction, a structure of the gas diffusionlayer 4, and a structure of the separator changes to carbon dioxide,thus leading to deterioration of the fuel cell.

Moreover, the following operations are also seen on the fuel electrode67. A reverse diffusion phenomenon moves the generation reaction waterfrom the oxidant electrode 34 side to the solid polymer electrolytemembrane 2, or the condensed water in the hydrogen which is humidifiedand supplied is, as the case may be, stored in the gas flow channel. Inthe case where the liquid water in a form of water drop is present inthe gas flow channel, no substantial problem is caused. However, in thecase where the liquid water in a form of membrane spreads widely tothereby cover a gas flow channel face of the gas diffusion layer 7, theliquid water prevents the hydrogen supply to the reaction surface, thuscausing portions with low hydrogen concentration. This may lead to thedeterioration of the fuel cell, like the above case on the oxidantelectrode 34 side.

The inconvenience caused by the liquid water in the gas flow channel isgenerally recognized, and a method for discharging the liquid water isimplemented. However, without the liquid water, the fuel cell isdeteriorated. That is, the deterioration phenomenon of the fuel cell(catalyst) is caused by a shortage of hydrogen in the fuel electrode 67,and therefore it is important to suppress occurrence of such a hydrogenshortage portion (for example, a portion of about 5% or less in volumeconcentration). Herein, a cause for lowering the hydrogen concentrationin the gas on the fuel electrode 67 side is that nitrogen contained inthe gas on the oxidant electrode 34 side permeates to the fuel electrode67 side. Thereby, it is necessary to properly obtain nitrogen permeationquantity. Therefore, at first, nitrogen permeation quantity (leaknitrogen quantity permeating through solid macromolecular membrane) perunit time relative to each of physical quantities (nitrogen partialpressure, temperature, and humidity) was checked throughexperimentations or simulations, with the results shown in FIG. 5 andFIG. 6.

FIG. 5 is an explanatory view showing leak nitrogen quantity relative tonitrogen partial pressure difference between the oxidant electrode 34and the fuel electrode 67. FIG. 6 is an explanatory view showing therelation between an ambient humidity and a leak nitrogen quantityaccording to ambient temperatures, where as denoted by a broken linearrow, the leak nitrogen quantity relative to the ambient humidity isincreased according to an increase in the ambient temperature, that is,Temp1, Temp2, Temp3 and Temp4. As shown in FIG. 5, the nitrogen quantitypermeating from the oxidant electrode 34 side to the fuel electrode 67side (leak nitrogen quantity) is larger as the nitrogen partial pressuredifference is larger. Moreover, as shown in FIG. 6, the nitrogenquantity permeating from the oxidant electrode 34 side to the fuelelectrode 67 side (leak nitrogen quantity) is larger as the humidity andtemperature at the fuel electrode 67 are higher.

As set forth above, in the fuel cell, the nitrogen permeated to the fuelelectrode 67 rides on the flow of the supplied hydrogen and then staysin such a manner as to be pushed into the downstream side (outlet side).Then, according to the present first embodiment, causing a forcedconvection current to agitate hydrogen with nitrogen suppressesoccurrence of the shortage portion where the hydrogen concentration islocally low.

FIG. 7 is an explanatory view schematically showing an agitation stateof hydrogen with the unreactive gas (mainly, nitrogen). As a method forimplementing agitation by the forced convection current, for example,the hydrogen pressure on the fuel electrode 67 side of the fuel cellstack 1 is rendered lower than the hydrogen supply pressure, to therebycause a predetermined differential pressure between inside and outsideof the fuel cell stack 1. Then, momentarily releasing the predetermineddifferential pressure can momentarily secure a large supply quantity(flow velocity) of hydrogen flowing into the fuel cell stack 1. Withthis, as shown in FIG. 7( a), the agitation between hydrogen andnitrogen becomes possible. When a turbulent flow is obtained, an effectof the agitation is larger although such effect depends on the size ofthe gas flow channel in the fuel cell. Moreover, even in the case of alaminar flow, since nitrogen is pushed to the capacity portion 12disposed at a downstream of the fuel cell stack 1 in the hydrogensystem, the gas in the fuel cell is replaced with hydrogen. Moreover,since the pressure is lowered in the entire gas flow channel, hydrogencan be distributed to the entire area of the gas flow channel until thepressure of the fuel electrode 67 becomes equal to the supply pressure.

For obtaining a constant differential pressure, it is also possible tosupply hydrogen to the fuel cell stack 1 in generating power whilemomentarily causing a large pressure. However, for more easily obtainingthe differential pressure, as shown in FIG. 7( b), the hydrogen supplyis stopped by means of the hydrogen pressure adjusting valve 11 (closingvalve operation) at a timing T1 while continuing the generation of thefuel cell stack 1. Then, a keeping time is set until a predetermineddifferential pressure (pressure width) ΔP1 is obtained, to therebysecure the differential pressure. After the predetermined differentialpressure ΔP1 is obtained (timing T2), hydrogen is supplied by means ofthe hydrogen pressure adjusting valve 11 (opening valve operation). Withthis, a large supply quantity (flow velocity) is momentarily caused,which can implement the agitation. Moreover, repeating the abovepressure change patterns (first pressure change pattern) at a period Cimplements the closing valve operation at a timing T3 and the openingvalve operation at a timing T4. With this, hydrogen can be pulsatorilysupplied. The differential pressure ΔP1 is, for example, in a range of 5kPa to 8 kPa. In view of the fuel cell stack characteristics, the gas'sagitation characteristics, and the like, experiments or simulations canset the optimum value of the differential pressure ΔP1. The differentialpressure ΔP1 necessary for the gas agitation is set smaller than thedifferential pressure necessary for an after-described liquid waterdischarge.

The above gas agitation can suppress the occurrence of the hydrogenshortage portion. However, in the case of the generation continuing fora long time, the generated water or condensed water is accumulated, thusblocking the fuel electrode 67 side gas flow channel in the fuel cell.Then, according to the present first embodiment, flowing hydrogen intothe fuel electrode 67 discharges the liquid water which blocks the gasflow channel out of the fuel cell.

FIG. 8 is an explanatory view showing a liquid water discharge state. Asa method of implementing the liquid water discharge by supplyinghydrogen, for example, the hydrogen pressure on the fuel electrode 67side of the fuel cell stack 1 is rendered lower than the hydrogen supplypressure, to thereby cause a predetermined differential pressure betweeninside and outside of the fuel cell stack 1. Then, momentarily releasingthe constant differential pressure can momentarily secure a large supplyquantity (flow velocity) of the fuel gas which flows into the fuel cellstack 1. With this, as shown in FIG. 8( a), the liquid water can bedischarged from the gas flow channel.

The differential pressure necessary for the liquid water discharge isrequired to be larger than the differential pressure necessary for theabove gas agitation. Meanwhile, the frequency required for the liquidwater discharge is lower than the frequency required for the gasagitation. Then, as shown in FIG. 8( b), a plurality of pressure changepatterns required for the gas agitation are implemented, then, at atiming Tm, the hydrogen supply is stopped by means of the hydrogenpressure adjusting valve 11 (closing valve operation). Then, a keepingtime is set until a predetermined differential pressure (pressure width)ΔP2 is obtained, to thereby secure the differential pressure. After thedifferential pressure ΔP2 is obtained (timing Tn), hydrogen is suppliedby means of the hydrogen pressure adjusting valve 11 (opening valveoperation). With this, a large flow velocity is momentarily caused, thusthe liquid water discharge can be implemented. Herein, the abovepressure change pattern (second pressure change pattern) is periodicallyrepeated, like the first pressure change pattern required for the gasagitation. However, compared with the first pressure pattern requiredfor the gas agitation, the second pressure change pattern required forthe liquid water discharge has lower implementation frequency. Thedifferential pressure ΔP2 is, for example, in a range of 20 kPa to 30kPa. In view of the fuel cell stack 1's characteristics, the liquidwater discharge characteristics and the like, experiments or simulationscan set the optimum value of the differential pressure ΔP2. Thedifferential pressure ΔP2 required for the liquid water discharge is setlarger than the differential pressure ΔP1 required for the above gasagitation.

Moreover, as shown in FIG. 8( c), a plurality of the pressure changepatterns required for the gas agitation are implemented and then, at thetiming Tm, the hydrogen supply is stopped by means of the hydrogenpressure adjusting valve 11 (closing valve operation). Then, a keepingtime is set until the predetermined differential pressure (pressurewidth) ΔP1 is obtained, to thereby secure the differential pressure.After the differential pressure ΔP1 is obtained (timing Tn), the openingdegree of the hydrogen pressure adjusting valve 11 is rendered largerthan that at the timing Tm, to thereby supply the hydrogen (openingvalve operation). With this, the gas is supplied at a pressure higherthan the pressure at the timing Tm, to thereby cause the predetermineddifferential pressure (pressure width) ΔP2 (timing To). Then, at atiming Tp, the hydrogen supply is stopped by means of the hydrogenpressure adjusting valve 11 (closing valve operation). Then, a keepingtime is set until the predetermined differential pressure (pressurewidth) ΔP2 is obtained, to thereby secure the differential pressure.After the differential pressure ΔP2 is obtained (timing Tq), hydrogen issupplied by means of the hydrogen pressure adjusting valve 11 (openingvalve operation). At that time, it is preferable that hydrogen issupplied at the opening degree same as that at the timing Tm. Then, at atiming Tr, the pressure returns to the same pressure as that at thetiming Tm. After the timing Tr, the pressure change patterns same asthose before the timing Tm are implemented. Even in the case of theabove operations, a large flow velocity is momentarily caused, so thatthe liquid water discharge can be implemented.

Moreover, as shown in FIG. 8( d), a plurality of pressure changepatterns required for the gas agitation are implemented and then, at thetiming Trn, the hydrogen supply is stopped by means of the hydrogenpressure adjusting valve 11 (closing valve operation). Then, a keepingtime is set until a differential pressure larger than the predetermineddifferential pressure (pressure width) ΔP1 is obtained. When adifferential pressure larger than the differential pressure ΔP1 isobtained (timing Tn), the opening degree of the hydrogen pressureadjusting valve 11 is rendered larger than that at the timing Tm, tothereby supply the hydrogen (opening valve operation). With this, thegas is supplied at the pressure higher than that at the timing Tm, tothereby cause the predetermined differential pressure (pressure width)ΔP2 (timing To). Next, at the timing Tp, the hydrogen supply is stoppedby means of the hydrogen pressure adjusting valve 11 (closing valveoperation). Then, a keeping time is set until a predetermineddifferential pressure (pressure width) ΔP3 is obtained, to therebysecure the differential pressure. Herein, it is preferable that thelower pressure limit at the obtaining of the differential pressure ΔP3is set to the lower pressure limit at the obtaining of the differentialpressure ΔP1. Next, after the differential pressure ΔP3 is obtained(timing Tq), hydrogen is supplied by means of the hydrogen pressureadjusting valve 11 (opening valve operation). At that time, it ispreferable that hydrogen is supplied at the opening degree same as thatat the timing Tm. Then, at the timing Tr, the pressure returns to thesame pressure as that at the timing Tm. After the timing Tr, thepressure change patterns same as those before the timing Tm areimplemented. Even when the above operations are implemented, a largeflow velocity can be momentarily caused, to thereby implement the liquidwater discharge.

As set forth above, according to the first embodiment, the controller 40controls the fuel gas supplier HS (10, 11, L1), to thereby supplyhydrogen to the fuel electrode 67 of the fuel cell stack 1, and based onthe first pressure change pattern which implements the pressure changeat the first pressure width ΔP1 and on the second pressure changepattern which implements the pressure change at the second pressurewidth ΔP2, the controller 40 periodically changes the hydrogen pressurein the fuel electrode 67 of the fuel cell stack 1.

With the above structure, the first pressure change pattern having asmall pressure width is used in addition to the second pressure changepattern, to thereby be able to agitate the fuel electrode 67 side gaswithout applying a large stress to the individual fuel cell of the fuelcell stack 1. With this, the fuel electrode 67 side gas can be madeeven. Thereby, the fuel cell stack 1's deterioration attributable to thepartial decrease of the hydrogen concentration can be suppressed.Moreover, providing the second pressure change pattern can discharge theliquid water and the like which cannot be discharged by the firstpressure change pattern. With this, the fuel cell stack 1'sdeterioration attributable to the liquid water can be suppressed.

Moreover, the fuel cell system 100 of the first embodiment adopts theclosed system where the fuel electrode off-gas discharged from the fuelelectrode 67 side of the fuel cell stack 1 is restricted. With the abovestructure, impurities are likely to decrease the hydrogen concentrationin the fuel electrode 67 side gas flow channel. However, implementingthe above control can make the fuel electrode 67 side gas even.

Moreover, according to the first embodiment, the controller 40implements the second pressure change pattern after implementing aplurality of first pressure change patterns. With the above structure,the frequency of applying a large stress to the individual cell of thefuel cell stack 1 can be decreased, while compatibly implementing thegas agitation and liquid water discharge on the fuel electrode 67 side.Moreover, since the implementation frequency of the first pressurechange pattern which implements the gas agitation is high, the gasagitation can effectively be implemented even when the generation iscontinuously implemented. With this, as shown in FIG. 9, even when thegeneration is continuously implemented, the current value in the powergeneration surface is substantially equal, thus the current value dropon the outlet side of the gas flow channel and the current concentrationon the inlet side of the gas flow channel can be suppressed.

Moreover, according to the first embodiment, the controller 40 stops thehydrogen supply to the fuel cell stack 1 in a state that the generationof the fuel cell stack 1 is implemented by supplying hydrogen at thepredetermined operation pressure, moreover, on a condition that thehydrogen pressure of the fuel electrode 67 is decreased by thepredetermined pressure width (ΔP1, ΔP2), the controller 40 restarts thehydrogen supply, to thereby change the hydrogen pressure in the fuelelectrode 67. With the above structure, the hydrogen pressure adjustingvalve 11 can easily implement the pressure change, so that a simplecontrol system can be accomplished.

Moreover, the fuel cell system 100 of the first embodiment has the fuelelectrode off-gas flow channel L2, the capacity portion 12 and the purgevalve 14. In this case, the capacity portion 12 functions as a space(capacity Rs: after-described FIG. 20) for storing the fuel electrodeoff-gas from the fuel electrode 67 side, that is, nitrogen or liquidwater. With this, though the fuel cell system 100 has substantially aclosed system, opening the purge valve 14 when necessary can alsodischarge the impurities (such as nitrogen which is relativelyincreased) outside. That is, the nitrogen leak is caused until thenitrogen partial pressure difference is removed. However, when thehydrogen concentration is to be kept at more than or equal to thepredetermined value on the fuel electrode 67 side, the flow ratecorresponding to the leak quantity can be discharged outside. Herein,the flow rate in this case is sufficiently small, thus unlikely to causean influence on the pressure change necessary for the gas agitation inthe fuel electrode 67, and in addition, diluting by the oxidantelectrode 34 off-gas can be easily implemented. However, the entirepressure on the fuel electrode 67 side may be increased such that thegeneration can be implemented even when the nitrogen partial pressure isbrought into an equilibrium state. In this case, a simple closed systemcan be adopted.

Moreover, when the hydrogen supply is stopped, the speed at which thehydrogen pressure in the fuel electrode 67 is decreased is determined bythe flow channel capacity in the fuel cell stack 1. When a rapidpressure decrease is not desired due to a request associated withcontrolling of the fuel cell system 100, changing the capacity of thehydrogen supply flow channel L1 to the fuel cell stack 1 or the capacityof the capacity portion 12 of the fuel electrode off-gas flow channel L2can control the pressure change time.

Second Embodiment

Hereinafter, the fuel cell system 100 according to the second embodimentof the present invention is to be set forth. The fuel cell system 100according to the second embodiment is different from the fuel cellsystem 100 according to the first embodiment in terms that the hydrogenquantity which is supplied to the fuel electrode 67 of the fuel cellstack 1 attributable to the pressure change by the pressure changepattern is variably set according to the operation condition of the fuelcell system 100. In addition, the structure of the fuel cell system 100according to the second embodiment is the same as that according to thefirst embodiment, therefore repeated explanations are to be omitted anddifferences are to be mainly set forth below.

FIG. 10 is a flowchart showing a control method of the fuel cell system100 according to the second embodiment of the present invention,specifically, showing process procedures of a method of supplyinghydrogen to the fuel electrode 67. The controller 40 implements theprocesses shown in this flowchart.

At first, at a step 1 (S1), the controller 40 detects the operationconditions of the fuel cell stack 1. The operation conditions detectedat this step 1 include an operation load of the fuel cell stack 1, anoperation temperature of the fuel cell stack 1, and an operationpressure of the fuel cell stack 1 (operation pressure of the oxidantelectrode 34). In view of the vehicle side required power specified fromthe vehicle speed or acceleration opening degree, the required power ofaccessories, and the like, the operation load of the fuel cell stack 1can be calculated. Moreover, the operation temperature of the fuel cellstack 1 can be detected by the stack temperature sensor 43. In terms ofthe operation pressure of the fuel cell stack 1, a certain standardvalue irrespective of the above operation load is set in advance, orvariable values according to the operation load are set in advance.Therefore, by referring to these values, the operation pressure of thefuel cell stack 1 can be detected.

At a step 2 (S2), the controller 40 determines whether or not theoperation condition thus detected at this time is changed compared tothe operation condition detected in advance. When the determination ispositive, that is, when the operation condition is changed, the routineproceeds to a step 3 (S3). Meanwhile, when the determination is negativein the step 2, that is, when the operation condition is not changed, theroutine skips the process at the step 3, to thereby proceed to a step 4(S4).

At the step 3, the controller 40 sets the pressure change pattern basedon the operation condition. As set forth according to the firstembodiment, the controller 40 implements a plurality of first pressurechange patterns necessary for the gas agitation and then implements thesecond pressure change pattern necessary for the liquid water discharge.By repeating the first and second pressure change patterns as one set,the controller 40 implements the hydrogen supply. By the way, in thesupply manner involving the pressure change, the hydrogen quantitysupplied to the fuel electrode 67 attributable to the pressure changepulsatorily varies, thus applying repeated loads to the solid polymerelectrolyte membrane 2, which acts as a stress. Then, in a scene wherethe cross leak from the oxidant electrode 34 is small, it is preferablethat the hydrogen quantity supplied to the fuel electrode 67attributable to the above pressure change is made small to therebydecrease the load applied to the solid polymer electrolyte membrane 2.Meanwhile, in a scene where the cross leak is large, it is preferable topositively implement the pressure change to thereby pulsatorily vary thehydrogen quantity supplied to the fuel electrode 67 attributable to thepressure change, thus implementing the gas agitation and liquid waterdischarge.

Ordinarily, the smaller the operation load of the fuel cell stack 1 is,the lower the operation temperature of the fuel cell stack 1 is, and thelower the operation pressure of the fuel cell stack 1 (specifically,operation pressure of the oxidant electrode 34) is; the smaller thecross leaked nitrogen quantity is. Then, when the operation condition ischanged according to any of the above cases, the hydrogen quantitysupplied to the fuel electrode 67 attributable to the pressure change isdecreased. On the contrary, the larger the operation load of the fuelcell stack 1 is, the higher the operation temperature of the fuel cellstack 1 is, and the higher the operation pressure of the fuel cell stack1 (specifically, operation pressure of the oxidant electrode 34) is; thelarger the cross leaked nitrogen quantity is. Then, when the operationcondition is changed according to any of the above cases, the hydrogenquantity supplied to the fuel electrode 67 attributable to the pressurechange is increased.

For setting small the hydrogen quantity supplied to the fuel electrode67 attributable to the pressure change, the basic control patterns areto be modified in the following manner.

As the first control method, as shown in FIG. 11, a valve closing time Tof the hydrogen pressure adjusting valve 11 is set longer than the valveclosing time of the basic control pattern. In other words, the basiccontrol pattern is to be so modified that the implementation period ofthe pressure change is set longer.

As the second control method, as shown in FIG. 12, differentialpressures (pressure widths) ΔP11, ΔP21 of the pressure control patternare set smaller than the differential pressures (pressure widths) ΔP1,ΔP2 of the pressure control pattern in the basic control pattern.

As the third control method, as shown in FIG. 13, the implementationfrequency of the second pressure change pattern (necessary for theliquid water discharge) relative to the first pressure change pattern(necessary for the gas agitation) is decreased compared with theimplementation frequency of the second pressure change pattern of thebasic control pattern.

Contrary to this, in the case of setting large the hydrogen quantitysupplied to the fuel electrode 67 attributable to the pressure change,each of the first to third control methods is to be controlled in theopposite direction.

According to the changed operation conditions, the controller 40modifies the basic control pattern based on any one of the first tothird control methods or a combination thereof. Then, the controller 40sets the thus modified control pattern as a present control pattern.

At the step 4, the controller 40 implements the hydrogen supply based onthe control pattern which is presently set.

At a step 5 (S5), the controller 40 determines whether or not to end theoperation of the fuel cell system 100. Specifically, the controller 40determines whether or not an off-signal is input from an ignitionswitch. When the determination is positive at the step 5, that is, whenthe operation of the fuel cell system 100 is to be ended, the presentcontrol is ended. Meanwhile, when the determination is negative at thestep 5, that is, when the operation of the fuel cell system 100 is notto be ended, the routine returns to the processes at the step 1.

As set forth above according to the second embodiment, with respect tothe fuel cell system 100, the hydrogen quantity supplied to the fuelelectrode 67 attributable to the pressure change is set small based onthe operation condition of the fuel cell system 100. With the abovestructure, while the gas agitation and liquid water discharge of thefuel electrode 67 are implemented, it is possible to decrease therepeated loads to the individual fuel cell of the fuel cell stack 1.

Third Embodiment

Hereinafter, the fuel cell system 100 according to the third embodimentof the present invention is to be set forth. Herein, the structure ofthe fuel cell system 100 according to the third embodiment is like thoseaccording to the first and second embodiments, therefore repeatedexplanations are to be omitted and differences are to be mainly setforth.

The controller 40 controls the fuel cell system 100 in the followingmanner. The controller 40 supplies air and hydrogen to the fuel cellstack 1, to thereby implement the generation by the fuel cell stack 1.In this case, the controller 40 supplies air and hydrogen such that thepressure of each of air and hydrogen which are supplied to the fuel cellstack 1 becomes a predetermined operation pressure. This operationpressure is set, for example, as a certain standard value irrespectiveof the power generated by the fuel cell stack 1, or set as variablevalues according to the power generated by the fuel cell stack 1.

According to the third embodiment, in terms of the air supply to theoxidant electrode 34, the controller 40 implements the pressure controlaccording to the predetermined operation pressure. Meanwhile, in termsof the hydrogen supply to the fuel electrode 67, the controller 40controls the supply-stop of hydrogen according to the control patternsfor implementing the pressure rise-fall within the range between anupper limit pressure P1 and a lower limit pressure P2. Then, thecontroller 40 repeats operations according to the control pattern, tothereby as shown in FIG. 14, supply hydrogen to the fuel electrode 67while periodically changing the hydrogen pressure in the fuel electrode67 of the fuel cell stack 1.

Specifically, on the condition that the hydrogen pressure of the fuelelectrode 67 reaches the upper limit pressure P1 and the hydrogenconcentration sufficient for implementing the generation is secured inthe fuel electrode 67, the controller 40 controls the hydrogen pressureadjusting valve 11 to the minimum opening degree, to thereby stop thehydrogen supply to the fuel cell stack 1. When from the fuel cell stack1 by way of the output takeout device 30, the controller 40 continues totake out a load current which corresponds to the load required by thefuel cell system 100, hydrogen is consumed by the generation reaction,to thereby lower the hydrogen pressure of the fuel electrode 67.

Next, on the condition that the hydrogen pressure of the fuel electrode67 is decreased to the lower limit pressure P2, the controller 40controls the hydrogen pressure adjusting valve 11 to the maximum openingdegree, to thereby restart the hydrogen supply to the fuel cell stack 1.With this, the hydrogen pressure in the fuel electrode 67 is increased.Then, on the condition that the hydrogen pressure reaches (comes backto) the upper limit pressure P1, the controller 40 controls the hydrogenpressure adjusting valve 11 to the minimum opening degree, to therebystop again the hydrogen supply. By repeating the above series ofprocesses as one-cycle control pattern, the controller 40 supplieshydrogen to the fuel electrode 67 of the fuel cell stack 1 whileperiodically changing the hydrogen pressure.

Herein, the upper limit pressure P1 and the lower limit pressure P2 arerespectively set based on, for example, a specified operation pressure.It is possible to monitor the hydrogen pressure of the fuel electrode 67of the fuel cell stack 1 by referring to values detected by the hydrogenpressure sensor 41. Moreover, for increasing the pressure, it is desiredthat the hydrogen pressure on the upstream side of the hydrogen pressureadjusting valve 11 is set sufficiently high in advance to therebyincrease a pressure-increasing speed as high as possible. For example,the pressure increase period from the lower limit pressure P2 to theupper limit pressure P1 is set to be in a range from 0.1 sec to about0.5 sec. Meanwhile, the time from the upper limit pressure P1 to thelower limit pressure P2 is in a range from 1 sec to about 10 sec,however, the above time depends on the upper limit pressure P1, thelower limit pressure P2 and the current value taken out of the fuel cellstack 1, that is, the hydrogen consumption speed.

In the hydrogen supply control involving the above periodical pressurerise-fall, as one of the features of the third embodiment, a firstkeeping time Tp1 and a second keeping time Tp2 for keeping the pressureof the fuel electrode 67 respectively at the upper limit pressure P1 andthe lower limit pressure P2 can be set to the control pattern. Thecontroller 40 can arbitrarily set the first keeping time Tp1 and secondkeeping time Tp2 in a range from zero to a predetermined value.

As shown in FIG. 15, the first keeping time Tp1 is a time for keepingthe pressure of the fuel electrode 67 at the upper limit pressure P1before implementing the first process for decreasing the pressure of thefuel electrode 67 from the upper limit pressure P1 to the lower limitpressure P2. Specifically, on the condition that the pressure of thefuel electrode 67 is decreased to the lower limit pressure P2, thecontroller 40 controls the opening degree Ot of the hydrogen pressureadjusting valve 11 to the maximum opening degree O1, to thereby restartthe hydrogen supply to the fuel cell stack 1, thus increasing thepressure of the fuel electrode 67. On the condition that the pressure ofthe fuel electrode 67 reaches the upper limit pressure P1, thecontroller 40 decreases the opening degree Ot of the hydrogen pressureadjusting valve 11 from the maximum opening degree O1 to a predeterminedopening degree, to thereby keep the pressure of the fuel electrode 67 atthe upper limit pressure P1. Then, on the condition that the firstkeeping time Tp1 elapsed from the timing at which the pressure of thefuel electrode 67 reaches the upper limit pressure P1, the controller 40controls the opening degree Ot of the hydrogen pressure adjusting valve11 to the minimum opening degree O2, to thereby stop the hydrogen supplyto the fuel cell stack 1.

Contrary to the above, as shown in FIG. 16, the second keeping time Tp2is a time for keeping the pressure of the fuel electrode 67 at the lowerlimit pressure P2 before implementing the second process for increasingthe hydrogen pressure of the fuel electrode 67 from the lower limitpressure P2 to the upper limit pressure P1. Specifically, on thecondition that the pressure of the fuel electrode 67 reaches the upperlimit pressure P1, the controller 40 controls the opening degree Ot ofthe hydrogen pressure adjusting valve 11 to the minimum opening degreeO2, to thereby stop the hydrogen supply to the fuel cell stack 1. On thecondition that the hydrogen pressure of the fuel electrode 67 isdeceased to the lower limit pressure P2, the controller 40 increases theopening degree Ot of the hydrogen pressure adjusting valve 11 from theminimum opening degree O2 to a predetermined opening degree, to therebykeep the pressure of the fuel electrode 67 at the lower limit pressureP2. Then, on the condition that the second keeping time Tp2 elapsed fromthe timing at which the pressure of the fuel electrode 67 reaches thelower limit pressure P2, the controller 40 controls the opening degreeOt of the hydrogen pressure adjusting valve 11 to the maximum openingdegree O1, to thereby restart the hydrogen supply to the fuel cell stack1, thus increasing the pressure of the fuel electrode 67.

FIG. 17 is an explanatory view showing the load relative to each of thefirst keeping time Tp1 and the second keeping time Tp2. For example, inthe case of a low load (for example, a condition of taking out the loadcurrent up to about ⅓ of a rated load current) as an operation scene ofthe fuel cell system 100, each of the first keeping time Tp1 and thesecond keeping time Tp2 is set at zero. Then, in the case of anintermediate load (for example, a condition of taking out the loadcurrent larger than about ⅓ to smaller than about ⅔ of the rated loadcurrent), the first keeping time Tp1 is set at zero while the secondkeeping time Tp2 is so set as to be increased as the load is higher withzero as a start point. Moreover, in the case of a high load (forexample, a condition of taking out the load current larger than or equalto about ⅔ of the rated load current), the first keeping time Tp1 is soset as to be increased as the load is higher with zero as a start pointwhile the second keeping time Tp2 is set constant. In this way, thecontroller 40 can determine the first keeping time Tp1 and the secondkeeping time Tp2 according to the load conditions. In other words,according to the load, the controller 40 can select whether to keep thepressure of the fuel electrode 67 at the upper limit pressure P1 or atthe lower limit pressure P2.

As set forth above, according to the third embodiment, as shown in FIG.17, when the required load is high (load current is large), thecontroller 40 increases the hydrogen supply quantity in theimplementation period of one control pattern, compared with when therequired load is low (load current is small). In the operation scenesuch as high load, the hydrogen consumption quantity is likely to belarge. Therefore, for covering the hydrogen supply, the number ofimplementations of the pressure rise-fall corresponding to one controlpattern may be increased. However, according to the third embodiment,the hydrogen supply quantity in the implementation period of one controlpattern is increased, thus the increase of the number of implementationsof the pressure rise-fall per unit time can be suppressed. With this,the stress applied to the fuel cell stack 1 or hydrogen-associatedcomponents can be relieved, thus the deterioration of the fuel cellsystem 100 can be suppressed.

Moreover, according to the third embodiment, as shown in FIG. 16, thefirst keeping time Tp1 for keeping the pressure of the fuel electrode 67at the upper limit pressure P1 before implementing the first process andthe second keeping time Tp2 for keeping the pressure of the fuelelectrode 67 at the lower limit pressure P2 before implementing thesecond process can be set to the control pattern. Then, the higher therequired load is, the longer the controller 40 sets the first keepingtime Tp1 or the second keeping time Tp2. With the required load beinghigh, the hydrogen consumption quantity is increased, to therebyincrease pressure drop speed in the first process. However, according tothe third embodiment, the larger the required load is, the longer thefirst keeping time Tp1 and second keeping time Tp2 are set. With this,the period from the timing at which the pressure of the fuel electrode67 reaches the upper limit pressure P1 to the timing at which thepressure of the fuel electrode 67 is returned from the lower limitpressure P2 to the upper limit pressure P1 can be set long. That is,setting long the first keeping time Tp1 and second keeping time Tp2 canelongate the implementation period of one control pattern, thussuppressing the increase in the number of implementations of thepressure rise-fall per unit time. With this, the stress applied to thefuel cell stack 1 or hydrogen-associated components can be relieved,thus suppressing the deterioration of the fuel cell system 100.

Especially, it is preferable that the higher the required load is, thelonger the controller 40 sets the first keeping time Tp1. With therequired load increased, as the case may be, it is difficult to securethe hydrogen partial pressure in the fuel electrode 67. Therefore,setting long the first keeping time Tp1 for the upper limit pressure P1can bring about an effect that the hydrogen partial pressure can besecured with ease even when the required load is high.

Moreover, according to the third embodiment, the higher the requiredload is in the required load's region from the low load to theintermediate load, the longer the second keeping time Tp2 is set (lowerin FIG. 17). From the low load to the intermediate load, the liquidwater is likely to be stored in the fuel electrode 67. Setting long thesecond keeping time Tp2 for the lower limit pressure P2 can enhanceaccuracy of implementing the liquid water discharge process. Moreover,it is preferable that the higher the required load is in the requiredload's region from the intermediate load to the high load, the longerthe controller 40 sets the first keeping time Tp1 (upper in FIG. 17).When the required load is increased, securing the hydrogen partialpressure in the fuel electrode 67 is, as the case may be, difficult.Therefore, setting long the first keeping time Tp1 for the upper limitpressure P1 can bring about an effect that the hydrogen partial pressurecan be secured with ease even when the required load is high.

In addition, as shown in FIG. 18, the hydrogen partial pressure may besecured in the following manner: the higher the impurity concentrationsuch as the nitrogen concentration in the fuel electrode 67 is (namely,immediately after the fuel cell system 100 is started), the longer thefirst keeping time Tp1 for keeping the upper limit pressure P1 is set.In this case, the longer the time until the fuel cell system 100restarts after stop, the higher the inactive gas concentration in thefuel electrode 67 is. Therefore, the first keeping time Tp1 for keepingthe upper limit pressure P1 may be made variable by measuring the stopperiod of the fuel cell system 100 or by measuring the nitrogenconcentration in the fuel electrode 67 at the start of the fuel cellsystem 100.

Moreover, in the fuel cell system 100 that adopts no idling (or idlereduction) which, at the low load and the like, temporarily stopsgeneration of the fuel cell stack 1 and allows traveling by means of apower of a secondary battery, the nitrogen concentration in the fuelelectrode 67 is high even immediately after the recovery from the noidling (or idle reduction). Then, in such a scene as well, the firstkeeping time Tp1 may be set long.

Fourth Embodiment

Hereinafter, the fuel cell system 100 according to the fourth embodimentof the present invention is to be set forth. Herein, the structure ofthe fuel cell system 100 according to the fourth embodiment is likethose according to the first to third embodiments, therefore repeatedexplanations are to be omitted. According to the fourth embodiment, amethod of setting the upper limit pressure P1 and lower limit pressureP2 is to be set forth.

First Setting Method

With respect to the first setting method, the upper limit pressure P1and the lower limit pressure P2 can be set according to the loadcurrent. Based on the vehicle speed, the acceleration operation quantityof the driver, and the information about the secondary battery, thecontroller 40 determines the fuel cell stack 1's target generation poweras the required load for the fuel cell system 100. Based on the targetgeneration power, the controller 40 calculates the load current which isa current value to be taken out from the fuel cell stack 1.

FIG. 19 is an explanatory view showing the upper limit pressure P1 andlower limit pressure P2 relative to the load current Ct. An operationpressure Psa for supplying the reactive gas necessary for taking out theload current Ct from the fuel cell stack 1 can be defined throughexperiments or simulations in view of the fuel cell system 100'scharacteristics such as the fuel cell stack 1, hydrogen system, airsystem and the like. Cr in FIG. 19 denotes a rated load current Cr{likewise, in an after-described FIG. 20( b)}.

For supplying air to the oxidant electrode 34, the operation pressurePsa is set as a target operation pressure.

Contrary to this, for supplying hydrogen to the fuel electrode 67, theupper limit pressure P1 and the lower limit pressure P2 are respectivelyset based on the operation pressure Psa. Herein, the upper limitpressure P1 and the lower limit pressure P2 are so set that the largerthe load current Ct is, the larger the differential pressure between theupper limit pressure P1 and the lower limit pressure P2 is, that is, thelarger the pressure change width in the gas supply operation is.

With the above structure, the higher the required load is, the more thehydrogen supply quantity in the implementation period of one controlpattern can be increased. With this, the increase in the number ofimplementations of the pressure rise-fall per unit time can besuppressed. With this, the deterioration of the fuel cell system 100 canbe suppressed.

Second Setting Method

As the second setting method, the upper limit pressure P1 and the lowerlimit pressure P2 may be set in view of the generation safety of thefuel cell stack 1. In the case of the low load, that is, when the loadcurrent is small, the differential pressure between the upper limitpressure P1 and the lower limit pressure P2 is so set as to berelatively small, for example, about 50 kPa. In this case, the averagehydrogen concentration in the individual fuel cell is about 40%.Contrary to this, in the case of the high load, that is, when the loadcurrent is large, the supply pressure on each of the oxidant electrode34 side and the fuel electrode 67 side is to be entirely increased sincethe gas pressure made larger can increase the generation efficiency. Inaddition, the difference between the upper limit pressure P1 and thelower limit pressure P2 is set at about 100 kPa. In this case, the fuelcell stack 1 is operated with the average hydrogen concentration ofabout 75% in the individual fuel cell.

According to the fourth embodiment which implements the periodicalpressure rise-fall, the atmosphere in the fuel cell stack 1 (fuelelectrode 67) is in a condition that the hydrogen concentration is lowat the timing of the lower limit pressure P2 while the hydrogenconcentration is high at the timing of the upper limit pressure P1. Thatis, increasing the pressure from the lower limit pressure P2 to theupper limit pressure P1 introduces a high hydrogen concentration gas tothe fuel electrode 67, to thereby push a low hydrogen concentration gasfrom the fuel cell stack 1 to the capacity portion 12. Moreover, thehigh hydrogen concentration gas agitates the gas in the fuel electrode67.

FIG. 20( a) and FIG. 20( b) are explanatory views schematically showingthe fuel electrode 67 side capacity Rs and the capacity Rt of thecapacity portion 12 in the fuel cell stack 1. For example, in the casewhere the upper limit pressure P1 is set at 200 kPa (absolute pressure)and the lower limit pressure P2 is set at 150 kPa (absolute pressure),the pressure ratio P1/P2 between the upper limit pressure P1 and thelower limit pressure P2 is about 1.33. In this case, as shown in FIG.20( a), the pressure increased from the lower limit pressure P2 to theupper limit pressure P1 allows an inflow of additional hydrogen to about¼ of the capacity (specifically, the capacity of the fuel cell stack 1and the capacity of the capacity portion 12) of the fuel system(=hydrogen system), that is, to 50% point of the fuel cell stack 1[hereinafter, this condition is expressed as hydrogen exchange ratio 0.5{refer to FIG. 20( b)}].

In the case of the low load, the hydrogen consumption speed is low,therefore, the hydrogen exchange ratio of around the above degree canimplement the generation of the fuel cell stack 1. In this scene, forexample, the hydrogen concentration of the time-averaged hydrogenelectrode off-gas is about 40%. Contrary to this, in the case of thehigh load, the pressure ratio P1/P2 (for example, 2 or more) whichreplaces the entire fuel electrode 67 of the fuel cell stack 1 with theadditional hydrogen is preferable, that is, the hydrogen exchange ratioof about 1 is preferable. Although the discharged hydrogen concentrationis preferably suppressed low, the hydrogen concentration greater than orequal to a predetermined value is necessary for stably implementing thegeneration (for example, about 75% or more is necessary) since thehydrogen consumption speed is high.

In the above cases, for adjusting the hydrogen concentration, the purgevalve 14 opens the fuel electrode off-gas flow channel L2. With this,such a minor amount of gas (flow rate) can be continuously orintermittently discharged from the purge valve 14 as not to prevent thehydrogen supply attributable to the periodical pressure rise-fall. Sincethe gas (flow rate) discharged from the purge valve 14 is minor, the gasis diluted by a cathode side exhaust (off gas) and then is safelydischarged out of the system. Opening of the purge valve 14 isimplemented for discharging the impurities (nitrogen or steam) from thefuel electrode 67, however, hydrogen is mixed in the fuel electrode 67.Therefore, it is preferable to effectively discharge the impurities bysuppressing the hydrogen discharge.

Then, according to the fourth embodiment, in the hydrogen supply, thepurge valve 14 is controlled to the open state corresponding to theprocess for increasing the hydrogen pressure from the lower limitpressure P2 to the upper limit pressure P1 (second process), to therebyopen the purge valve 14 (purge process). Specifically, the controller 40monitors the pressure of the fuel electrode 67 of the fuel cell stack 1,and then controls the purge valve 14 to the open state according to atiming at which the monitored pressure reaches the lower limit pressureP2, moreover, the controller 40 controls the purge valve 14 to theclosed state according to a timing at which the monitored pressurereaches the upper limit pressure P1 (basic control pattern). With this,the low hydrogen concentration gas is pushed into the capacity portion12 from the fuel cell stack 1, and then, the low hydrogen concentrationgas is discharged from the capacity portion 12 by way of the purge valve14 before the high concentration hydrogen gas reaches the purge valve14. With this, many impurities can be efficiently discharged.

However, the opening-closing control of the purge valve 14 is notlimited to this basic control pattern. Provided that the purge valve 14is so controlled to the open state as to include at least the process ofincreasing the pressure from the lower limit pressure P2 to the upperlimit pressure P1 (second process), the opening-closing control of thepurge valve 14 is sufficient. Therefore, the timing for controlling thepurge valve 14 to the closed state can be modified also to a timingwhich is later than the timing (hereinafter, referred to as “basicclosing timing”) at which the hydrogen pressure reaches the upper limitpressure P1. For example, in view of a diffusion speed, a boundarybetween the high concentration hydrogen and the low concentrationhydrogen can be determined as a constant face within a short time. Then,with respect to the fuel cell stack 1 and capacity portion 12 during thehydrogen supply operation, how long time it takes for a boundary face(what is called a hydrogen front) to reach and up to which position theboundary face reaches are to be estimated in advance through experimentsor simulations. Then, until the boundary face reaches the purge valve14, the timing of controlling the purge valve 14 to the closed state canbe further delayed than the basic closing timing.

Moreover, it is not necessary to implement the purge treatment for eachimplementation of the control pattern, specifically, for every pressureincreasing process (second process). For example, on the condition thatthe hydrogen concentration in the fuel electrode 67 reaches less than orequal to a predetermined determination threshold, the purge valve 14 maybe opened according to the subsequent pressure increasing process.

Moreover, since the liquid water also is regarded as a factor fordisturbing the generation reaction, the liquid water can also bedischarged. However, compared with the presence of the inactive gas, thetime for the liquid water to cause an influence is longer. Therefore, itis preferable to implement the liquid water discharge treatment once ina plurality of periodical pressure rise-fall operations or atpredetermined time intervals, instead of every periodical pressurerise-fall operation. It is sufficient that the liquid water be removedfrom inside the fuel cell stack 1. Therefore, the discharging of theliquid water from the fuel cell stack 1 to the capacity portion 12 is tobe taken into account. In this case, since increase of the flow velocityis necessary, the differential pressure between the upper limit pressureP1 and the lower limit pressure P2 is preferably set about 100 kPa.

Moreover, in terms of the upper limit pressure P1 and the lower limitpressure P2, the following additional methods can be set in addition tothe thus-far described method of varying the upper limit pressure P1 andthe lower limit pressure P2 according to the required load.

At first, as the first additional method, the upper limit pressure P1and the lower limit pressure P2 may be set according to an allowabledifferential pressure between the oxidant electrode 34 and fuelelectrode 67 in the fuel cell.

Moreover, as the second additional method, in the fuel cell system 100for implementing the purge treatment for discharging the inactive gasaccumulated in the fuel electrode 67, the upper limit pressure P1 andthe lower limit pressure P2 may be so restricted as to secure theminimum pressure for securely implementing the purging.

Moreover, as the third additional method, the upper limit pressure P1 isset larger as the nitrogen concentration (impurity concentration) in thefuel electrode 67 is higher, and the lower limit pressure P2 is set to asmall value in a condition that the liquid water staying quantity orliquid water generation quantity in the fuel electrode 67 is expected tobe large. With this, a large differential pressure is already securedwhen it is determined that the liquid water is actually stored, tothereby be able to securely implement the liquid water discharge.

Moreover, as the fourth additional method, in a scene where the liquidwater quantity staying in the fuel cell stack 1 is assumed to be large,as shown in FIG. 21, the upper limit pressure P1 and the lower limitpressure P2 are so set as to allow the pressure ratio (P1/P2) betweenthe upper limit pressure P1 and the lower limit pressure P2 istemporarily large (P1 w/P2w). The pressure width ΔP2 P1w−P2w) necessaryfor discharging the liquid water in the fuel electrode 67 is, forexample, more than or equal to 100 kPa, and the pressure width ΔP1(=P1−P2) for discharging the inactive gas in the fuel electrode 67 is,for example, more than or equal to 50 kPa. As stated above, since thepressure widths of the two are different from each other, the upperlimit pressure P1 and the lower limit pressure P2 are set as describedabove in view of the liquid water discharge.

Herein, when the upper limit pressure P1 is set high, that is, to P1w,as stated in the third and fourth additional methods, the speed oflowering the pressure from the upper limit pressure P1 to the lowerlimit pressure P2 is decreased since the hydrogen consumption speed issmall in the low load region. In this case, since the time is requireduntil the pressure reaches the lower limit pressure P2, as the case maybe, the second process for increasing the pressure from the lower limitpressure P2 to the upper limit pressure P1 cannot be implemented for awhile.

Then, as shown in FIG. 22, when the upper limit pressure P1 is set high(for example, pressure P1w) in the low load condition, it is permittedthat the controller 40 temporarily increases the current taken out fromthe fuel cell stack 1, to thereby increase the pressure drop speed. Forexample, when the current is not increased, the time required fordecreasing the pressure from the upper limit pressure P1w to the lowerlimit pressure P2 is a time Tm2. Meanwhile, increasing the currentallows the time required for decreasing the pressure from the upperlimit pressure P1w to the lower limit pressure P2 to be a time Tm3(=Tm1) which is shorter than the time Tm2. With this, an interference tothe pressure rise-fall control for the inactive gas discharge or aninterference to the pressure rise-fall control for the subsequent liquidwater discharge can be suppressed.

In addition, when the generation condition may possibly be made unstableattributable to a temporary increase of the current taken out of thefuel cell stack 1, which temporary increase is implemented in such ascene that the voltage of the fuel cell stack 1 is lowered, or in thecase where the charge level of the secondary battery for storing thetaken-out current is high, another method may be used for increasing thepressure drop speed, instead of the method of increasing the taken-outcurrent.

As the other method for increasing the pressure drop speed, for example,the flow rate of the fuel electrode off-gas discharged from the purgevalve 14 is to be increased. Moreover, the pressure drop speed may beincreased by enlarging the capacity of the fuel electrode 67. As amethod for enlarging the capacity of the fuel electrode 67, the liquidwater control level in the fuel electrode 67 is lowered, to therebydischarge the liquid water in the fuel electrode 67.

In addition, as a method of estimating the liquid water staying quantityin the fuel electrode 67, an estimation method by accumulating the loadcurrent based on the feature that the liquid water generation quantityis substantially proportional to the load current can be considered.Moreover, the liquid water staying quantity may be estimated by the timeelapsed from the timing of the liquid water discharge implemented inadvance. Moreover, by measuring the voltage of the fuel cell,estimating, based on the fuel cell's voltage which is abnormallylowered, that the liquid water staying quantity is large is allowed.Moreover, in the estimation of the liquid water staying quantity, thetemperature of the coolant water for cooling the fuel cell stack 1 canbe used for correcting the liquid water staying quantity. The reasontherefor is that even when the load current is the same, the lower thecoolant water temperature is, the more the liquid water (quantity)stays. Likewise, the number of pressure pulsations or the cathode's airquantity can also correct the liquid water staying quantity.

Fifth Embodiment

Hereinafter, the fuel cell system 100 according to the fifth embodimentof the present invention is to be set forth. According to the thirdembodiment, the ordinary operation process for implementing thegeneration according to the load current in the fuel cell stack 1 hasbeen set forth. Meanwhile, according to the fifth embodiment, theprocess of each of at the start and stop of the fuel cell system 100 isto be set forth. Herein, the structure of the fuel cell system 100according to the fifth embodiment is like those according to the firstto fourth embodiments, therefore repeated explanations are to be omittedand differences are to be mainly set forth.

Start Process

At first, the start process of the fuel cell system 100 is to be setforth. In the case where after the stop of the fuel cell system 100, thefuel cell stack 1 is left as it is for a while instead of being startedimmediately, the low hydrogen concentration gas is filled in the fuelelectrode 67. In the case of starting the system 10 in the above state,the low hydrogen concentration gas is to be discharged from the fuelelectrode 67 of the fuel cell stack 1. Therefore, the high hydrogenconcentration gas is to be momentarily supplied from the fuel tank 10 ata predetermined starting upper limit pressure, to thereby increase thegas pressure in the fuel electrode 67. In this case, the purge valve 14is also controlled to the open state. With this, the passage of thehydrogen front which is the boundary face between the low hydrogenconcentration gas and the high hydrogen concentration gas can beaccelerated, and also the hydrogen front can be pushed out of the fuelelectrode 67.

Then, before the timing at which the hydrogen front reaches the purgevalve 14, the hydrogen pressure adjusting valve 11 and the purge valve14 are controlled to the closed state, to thereby implement thegeneration and consume hydrogen, thus reducing the hydrogen pressure inthe fuel electrode 67. Then, when the hydrogen pressure reaches apredetermined starting lower limit pressure, the hydrogen pressure isagain increased to the predetermined starting upper limit pressure.Then, the above pressure rise-fall operations are to be repeated untilthe hydrogen concentration of the fuel electrode 67 of the fuel cellstack 1 reaches the predetermined average hydrogen concentration.

In addition, an actual vehicle, as the case may be, starts moving duringthe period that the above start process is being implemented. In thiscase, the output from the installed secondary battery may be used.

Stop Process

Then, the stop process of the fuel cell system 100 is to be set forth.As a start scene after stopping the fuel cell system 100, a lowtemperature environment is assumed. In this case, when the liquid wateris present in the fuel cell stack 1, hydrogen pressure adjusting valve11, discharge water valve 13, purge valve 14 and the like at the stop ofthe fuel cell system 100, as the case may be, freezing and the likedisenables starting of the fuel cell system 100. Therefore, it isnecessary to establish a process for removing the liquid water at thestop of the fuel cell system 100. At first, air is to be supplied to theoxidant electrode 34 while implementing the generation in the low loadcondition. On the fuel electrode 67 side, the pressure rise-falloperations are to be repeatedly implemented according to the controlpattern, like the third embodiment. In this case, for example, with theupper limit pressure P1 at 200 kPa (absolute pressure) and the lowerlimit pressure P2 at 101.3 kPa, sufficient values should be set inadvance for discharging the liquid water from the fuel electrode 67.Moreover, the number of repetitions of pressure rise-fall operations forsufficiently discharging the liquid water are to be obtained in advancethrough experiments or simulations. Based the thus obtained numbers, thepressure rise-fall operations should be repeated. With this, thegeneration is ended.

Then, with the discharge water valve 13 controlled to the open state,the discharge liquid water from the fuel cell stack 1 to the capacityportion 12 is discharged. Then, the power which was generatedimmediately before the discharge operation is used, to thereby operateheating devices such as heater and the like after the above dischargeoperation, thus heating the purge valve 14 and the discharge water valve13, to thereby dry the discharge liquid water.

According to the fifth embodiment, in the fuel cell system 100, the stopprocess can accomplish startability at the start, in addition, even theprocess at the start can discharge impurities more preferentially thanhydrogen.

The entire contents of the Japanese Patent Application Laid-Open No.2008-298191 (filed on Nov. 21, 2008) and Japanese Patent ApplicationLaid-Open No. 2008-302465 (filed on Nov. 27, 2008) are incorporatedherein by reference in order to take protection against translationerrors or omitted portions.

As set forth above, the contents of the present invention have been setforth based on the embodiments. However, it is obvious to a personskilled in that art that the present invention is not limited to theabove embodiments and various modifications and improvements thereof areallowed.

INDUSTRIAL APPLICABILITY

According to the present invention, based on the first pressure changepattern for implementing the pressure change at the first pressurewidth, the pressure of the fuel gas in the fuel electrode isperiodically changed, to thereby be able to agitate the fuel electrodeside gas. With this, the fuel electrode side gas can be made even.

1. A fuel cell system comprising: a fuel cell configured to generateelectric power by causing an electrochemical reaction between an oxidantgas supplied to an oxidant electrode and a fuel gas supplied to a fuelelectrode; a fuel gas supplier configured to supply the fuel gas to thefuel electrode of the fuel cell; an output takeout device configured totake out an output from the fuel cell; and a controller configured tocontrol the output takeout device to thereby take out from the fuelcell, an output corresponding to a required load required for the fuelcell system, and to control the fuel gas supplier to thereby supply thefuel gas to the fuel electrode in such a manner as to change a pressureof the fuel gas at the fuel electrode with a predetermined pressurechange range, wherein the controller is programmed to set the pressurechange range such that the pressure change range in a case where therequired load is high is larger than the pressure change range in a casewhere the required load is low.
 2. The fuel cell system according toclaim 1, wherein the controller sets an operation pressure of the fuelcell such that the higher the required load is, the higher the operationpressure is.
 3. The fuel cell system according to claim 1, wherein thecontroller sets an upper limit pressure and a lower limit pressure ofthe pressure of the fuel gas at the fuel electrode based on an operationpressure of the fuel cell, and change the pressure of the fuel gas atthe fuel electrode between the upper limit pressure and the lower limitpressure to thereby change the pressure of the fuel gas at the fuelelectrode with the predetermined pressure change range.
 4. The fuel cellsystem according to claim 3, wherein a rate of increase of the lowerlimit pressure relative to an increase of the required load is set suchthat the rate of increase of the lower limit pressure in a case wherethe required load is high is larger than the rate of increase of thelower limit pressure in a case where the required load is low.
 5. A fuelcell system comprising: a fuel cell configured to generate electricpower by causing an electrochemical reaction according to a load of thefuel cell system between an oxidant gas supplied to an oxidant electrodeand a fuel gas supplied to a fuel electrode and to consume the fuel gasin the fuel electrode; a non-recirculating type fuel gas systemcomprising: a fuel gas supplier configured to supply the fuel gas to aninlet of the fuel electrode of the fuel cell: a capacity device providedon an outlet side of the fuel electrode of the fuel cell; and a purgevalve provided on the outlet side of the fuel electrode of the fuelcell; and a pressure increase-decrease controller programmed to have apressure of the fuel gas at the fuel electrode increased with anincrease of the load, while controlling the fuel gas supplier toincrease/decrease the pressure of the fuel gas at the fuel electrodewith a predetermined pressure increase/decrease range at a given load,wherein the pressure increase-decrease controller sets, in a case wherethe load is high, a large pressure increase/decrease range as comparedto a case where the load is low.